Many commercial applications require the separation of two fluids, e.g., gasses or liquids, by using a membrane. The membrane is selected to mediate the interaction of the two fluids. For example, in a hydrogen purification system, the membrane may mediate the interaction of a hydrogen-rich stream at high pressure from a pure hydrogen stream at lower pressure. In this example, the membrane may be constructed from a material that allows hydrogen diffusion at a higher rate than other components of the hydrogen-rich stream. In another example, a membrane in a fuel cell may mediate the interaction of an oxygen-containing fluid with a fuel-containing fluid. The fuel cell membrane may include multiple layers that allow one or more types of ions to pass through the membrane to oxidize the fuel, while extracting electrical energy from that reaction.
To improve performance and decrease system size, it is often desirable to miniaturize membrane-containing systems. However, the materials that can readily be manufactured in a miniaturized fashion are not always compatible with the materials that are optimal for membrane functionality. Integration of these two materials sets may result in substantial stresses in the membrane. Stress may be induced by, for example, different thermal expansion rates of the membrane and a supporting structure.
A specific example of a need for miniaturization may be found in the area of batteries and fuel cells. The proliferation of portable electronics, including cellular telephones and laptop computers, has increased the demand on power storage devices, such as batteries. Fuel cells may be used to increase energy storage available in comparison to batteries. The fuel cell system, however, must be miniaturized to fit within the small form-factors of existing batteries. One example of a type of fuel cell is a solid oxide fuel cell, which is known to have high efficiency. One common miniaturization technique employs silicon (Si) substrates and integrated circuit manufacturing technologies. Silicon expands at a rate of approximately 4 micrometers per meter per degree Celsius (μm/m/° C.). Conventional solid oxide fuel cells use materials that expand at a rate of approximately 10 μm/m/° C., and operate at temperatures of about 800° C. Combining a conventional solid oxide fuel cell membrane with conventional silicon manufacturing may cause a significant expansion mismatch of approximately 0.5%, leading to highly stressed membranes. Additional factors contributing to the stress may include intrinsic stress of the thin film as deposited, tensile or compressive stress induced by sintering or other thermal processing, and chemical modifications inducing tensile or compressive stresses. High stresses in the thin-film membranes may cause mechanical failure of the film or the stress level may undesirably change material properties.
Design of fuel cell membranes, such as yttrium-stabilized zirconia (YSZ) on Si substrates, may require a free-standing YSZ thin film to stretch over a 1 millimeter (mm) to a 1 centimeter (cm) diameter. These membranes may fail because these membranes may be pliable or prone to buckling. Furthermore, YSZ membranes may also fail when cooled into the tensile state because of crack propagation.